A motor vehicle, especially a large one like a heavy truck or highway tractor, may be equipped with one or more mechanisms in its powertrain that when operated can apply a load on the powertrain for decelerating the vehicle. Any controllable device or mechanism that will produce drivetrain losses resulting in vehicle deceleration can serve as a retarder during certain operating conditions. Examples of such mechanisms are devices like exhaust brakes, engine brakes, and driveline or transmission retarders. Downshifting of a transmission can also decelerate a vehicle. An engine cooling fan that can be selectively connected to and disconnected from the engine could be used as a retarder during certain conditions. Still other devices and mechanisms include air intake throttle valves, electrical loads, and energy storage devices that are not associated with hybrid vehicle operation.
Large motor vehicles typically have service, or foundation, brakes at individual road wheels, and those wheel brakes are the primary means for decelerating the vehicle. Wheel brakes that apply friction forces to rotating wheels create torque that opposes wheel rotation. That torque creates forces at the interfaces between the wheels' tires and road surface opposing the direction of vehicle motion, thereby decelerating the vehicle. However, the friction forces that act on the rotating road wheels generate heat that raises brake temperature. The heat and wear created by numerous applications of the service brakes eventually degrade the effectiveness of brake linings or brake pads, necessitating their replacement.